A. Field of Invention
This invention relates to devices for measuring the three force and three moment components produced by applying a load to such a device; specifically, measurement devices utilizing strain gages and the placement and orientation of the strain gages on the measuring device.
B. Description of the Prior Art
Multi-component load transducers are typically designed by combining various flexure elements into one body, where each flexure element incorporates strain gages to measure a component of force or moment.
Alternatively, a cylindrical tubular design can be used where forces and moments are measured by gages specially positioned and oriented to isolate the individual load components to be measured. (Cunningham and Brown, 1952) (Smith, 1970). The traditional method of isolating the individual load components to be measured had the advantage where each output of the transducer corresponded to one of the force or moment components to be determined. Therefore, the conversion from measured electrical output signals to loads was a simple procedure.
Another approach measures the three force components transmitted through the neck of a hip prosthesis using three strain gages attached to the prosthesis with no specific orientation. (Bergmann, Journal of Biomechanics, 1988). It would appear from the literature that the strain gages are oriented and placed randomly. This results in a more complicated measurement conversion equation. Random orientation and placement has been used in multiple devices. The theory is that, as long as each gage measures a quantity independent from the others, random orientation and placement is possible. Unfortunately, random orientation and placement has the disadvantage that some channels may not have their optimum sensitivities.
After the advent of computers, difficult conversions as above are normally done by software. Thus, complicated conversion equations no longer pose a problem, yet the sensitivity problems remain.
The present invention eliminates the sensitivity disadvantages by positioning one set of strain gages in approximately a less than sixty degree orientation to the long axis of the cylindrical or tubular load cell, and another set of gages approximately between forty-five and one hundred-twenty degrees from the first set and approximately less than sixty degrees from the long axis of the cell. This configuration optimizes output sensitivities, and eliminates the need for precision placement of the gages. A computer or other calculation means shall be used to determine the components and compensate for drift and noise.